The present invention relates to an improved thermosensitive recording material comprising a support material and a thermosensitive coloring layer formed thereon, in which thermosensitive coloring layer, colored images are formed by the reaction between a colorless or light-colored leuco dye and a color-developer which induces color information in the leuco dye when heat is applied thereto. More particularly, it relates to a thermosensitive recording material of the above-mentioned type which is particularly improved with respect to the thermosensitive recording performance, with preservability and stability of the developed images with high resistance to moisture, heat, light, chemicals and oils, and the whiteness of the background.
A conventional thermosensitive recording material comprises a support material, such as paper, synthetic paper or a plastic film, and a thermosensitive coloring layer formed thereon which consists essentially of a composition which is colored upon application of heat thereto. On the thermosensitive recording material, colored images are formed by application of heat by use of a thermal head or a thermal pen, or by application of laser beams thereto. Recording materials of this kind are widely used for making copies from books and documents or as output charts for computers and facsimile apparatus, tickets for use with vending machines, labels and other charts for recording, since in comparison with other recording materials, they have the advantages that complex recording steps, such as development and image fixing, are unnecessary, recorded images can be quickly obtained by a comparatively simple apparatus, no noise is generated during operation, and they are free from a problem of air pollution and cheaper than other recording materials.
A thermosensitive composition for color formation employed in such a thermosensitive recording material is generally composed of a coloring agent and a color developer which is capable of inducing color formation in the coloring agent upon application of the heat thereto.
As the coloring agent, for example, colorless or light-colored leuco dyes having lactone rings, lactam rings or spyropyran rings are employed.
As the color developer, a wide variety of acidic materials such as organic acids and phenolic materials are employed. The recording materials in which such a coloring agent and a color developer are used in combination have the advantages that the color tone of the images is clear, the whiteness of the background is high and the images are excellent in weathering resistance. Therefore they are widely used.
Recently other conventional recording systems are being replaced by thermosensitive recording systems and a demand for thermosensitive systems in increasing. In accordance with this trend, further improvement of the quality of the thermosensitive recording materials for use with the thermosensitive recording systems is desired. In particular, a thermosensitive recording material which is improved on the whitenes of the background and the preservability of the developed images is desired. In order to improve the preservability and stability of the developed image, it is proposed that a mixture of two or more leuco dyes be employed, for instance, in Japanese Laid-Open Patent Application No. 54-109454, and a mixture of an organic acid and a phenolic material be employed as a color developer, for instance, in Japanese Patent Publication No. 51-43386. In the case of these prior art methods, when the preservability and stability of the developed images are improved, the fogging of the background takes place, resulting in that the whiteness of the background is decreased. On the other hand, when the whiteness of developed images is increased by preventing the fogging of the background from taking place, the preservability and stability of the images inevitably decreases.